[PATCH kernel] powerpc/64: Add UADDR64 relocation support
Alexey Kardashevskiy
aik at ozlabs.ru
Tue Feb 1 11:25:31 AEDT 2022
On 1/31/22 17:38, Christophe Leroy wrote:
>
>
> Le 31/01/2022 à 05:14, Alexey Kardashevskiy a écrit :
>> When ld detects unaligned relocations, it emits R_PPC64_UADDR64
>> relocations instead of R_PPC64_RELATIVE. Currently R_PPC64_UADDR64 are
>> detected by arch/powerpc/tools/relocs_check.sh and expected not to work.
>> Below is a simple chunk to trigger this behaviour:
>
> According to relocs_check.sh, this is expected to happen only with
> binutils < 2.19. Today minimum binutils version is 2.23
>
> Have you observed this problem with newer version of binutils ?
Oh yeah. 2.36.1. And the toolchain folks explained internally that this
is correct behavior and this was a ticking bomb which exploded now and
the kernel has to deal with it.
>>
>> \#pragma GCC push_options
>> \#pragma GCC optimize ("O0")
>
> AFAIU Linux Kernel is always built with O2
Correct. Even O1 hides this.
> Have you observed the problem with O2 ?
Yes, I see it once I enable CONFIG_PRINTK_INDEX (this is how it was
spotted with my particular config, there is still a fair chance that
this config option does not cause UADDR64 always) but I did not debug
with it enabled as pretty much every single __func__ passed to printk
caused unaligned relocation (tens of thousands). Note that this
particular case can be fixed by removing __packed from "struct pi_entry"
(== re-arm the bomb). Thanks,
>
>
>> struct entry {
>> const char *file;
>> int line;
>> } __attribute__((packed));
>> static const struct entry e1 = { .file = __FILE__, .line = __LINE__ };
>> static const struct entry e2 = { .file = __FILE__, .line = __LINE__ };
>> ...
>> prom_printf("e1=%s %lx %lx\n", e1.file, (unsigned long) e1.file, mfmsr());
>> prom_printf("e2=%s %lx\n", e2.file, (unsigned long) e2.file);
>> \#pragma GCC pop_options
>>
>>
>> This adds support for UADDR64 for 64bit. This reuses __dynamic_symtab
>> from the 32bit which supports more relocation types already.
>>
>> This adds a workaround for the number of relocations as the DT_RELACOUNT
>> ELF Dynamic Array Tag does not include relocations other than
>> R_PPC64_RELATIVE. This instead iterates over the entire .rela.dyn section.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>> ---
>>
>> Tested via qemu gdb stub (the kernel is loaded at 0x400000).
>>
>> Disasm:
>>
>> c000000001a804d0 <e1>:
>> c000000001a804d0: b0 04 a8 01 .long 0x1a804b0
>> c000000001a804d0: R_PPC64_RELATIVE *ABS*-0x3ffffffffe57fb50
>> c000000001a804d4: 00 00 00 c0 lfs f0,0(0)
>> c000000001a804d8: fa 08 00 00 .long 0x8fa
>>
>> c000000001a804dc <e2>:
>> ...
>> c000000001a804dc: R_PPC64_UADDR64 .rodata+0x4b0
>>
>> Before relocation:
>>
>>>>> p *(unsigned long *) 0x1e804d0
>> $1 = 0xc000000001a804b0
>>>>> p *(unsigned long *) 0x1e804dc
>> $2 = 0x0
>>
>> After:
>>>>> p *(unsigned long *) 0x1e804d0
>> $1 = 0x1e804b0
>>>>> p *(unsigned long *) 0x1e804dc
>> $2 = 0x1e804b0
>> ---
>> arch/powerpc/kernel/reloc_64.S | 47 +++++++++++++++++++++++++-----
>> arch/powerpc/kernel/vmlinux.lds.S | 3 +-
>> arch/powerpc/tools/relocs_check.sh | 6 ----
>> 3 files changed, 41 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/reloc_64.S b/arch/powerpc/kernel/reloc_64.S
>> index 02d4719bf43a..a91175723d9d 100644
>> --- a/arch/powerpc/kernel/reloc_64.S
>> +++ b/arch/powerpc/kernel/reloc_64.S
>> @@ -10,6 +10,7 @@
>> RELA = 7
>> RELACOUNT = 0x6ffffff9
>> R_PPC64_RELATIVE = 22
>> +R_PPC64_UADDR64 = 43
>>
>> /*
>> * r3 = desired final address of kernel
>> @@ -25,6 +26,8 @@ _GLOBAL(relocate)
>> add r9,r9,r12 /* r9 has runtime addr of .rela.dyn section */
>> ld r10,(p_st - 0b)(r12)
>> add r10,r10,r12 /* r10 has runtime addr of _stext */
>> + ld r13,(p_sym - 0b)(r12)
>> + add r13,r13,r12 /* r13 has runtime addr of .dynsym */
>>
>> /*
>> * Scan the dynamic section for the RELA and RELACOUNT entries.
>> @@ -46,8 +49,8 @@ _GLOBAL(relocate)
>> b 1b
>> 4: cmpdi r7,0 /* check we have both RELA and RELACOUNT */
>> cmpdi cr1,r8,0
>> - beq 6f
>> - beq cr1,6f
>> + beq 9f
>> + beq cr1,9f
>>
>> /*
>> * Work out linktime address of _stext and hence the
>> @@ -60,25 +63,55 @@ _GLOBAL(relocate)
>> subf r10,r7,r10
>> subf r3,r10,r3 /* final_offset */
>>
>> + /*
>> + * FIXME
>> + * Here r8 is a number of relocations in .rela.dyn.
>> + * When ld issues UADDR64 relocations, they end up at the end
>> + * of the .rela.dyn section. However RELACOUNT does not include
>> + * them so the loop below is going to finish after the last
>> + * R_PPC64_RELATIVE as they normally go first.
>> + * Work out the size of .rela.dyn at compile time.
>> + */
>> + ld r8,(p_rela_end - 0b)(r12)
>> + ld r18,(p_rela - 0b)(r12)
>> + sub r8,r8,r18
>> + li r18,24 /* 24 == sizeof(elf64_rela) */
>> + divd r8,r8,r18
>> +
>> /*
>> * Run through the list of relocations and process the
>> - * R_PPC64_RELATIVE ones.
>> + * R_PPC64_RELATIVE and R_PPC64_UADDR64 ones.
>> */
>> mtctr r8
>> -5: ld r0,8(9) /* ELF64_R_TYPE(reloc->r_info) */
>> +5: lwa r0,8(r9) /* ELF64_R_TYPE(reloc->r_info) */
>> cmpdi r0,R_PPC64_RELATIVE
>> bne 6f
>> ld r6,0(r9) /* reloc->r_offset */
>> ld r0,16(r9) /* reloc->r_addend */
>> - add r0,r0,r3
>> + b 7f
>> +
>> +6: cmpdi r0,R_PPC64_UADDR64
>> + bne 8f
>> + ld r6,0(r9)
>> + ld r0,16(r9)
>> + lwa r14,12(r9) /* ELF64_R_SYM(reloc->r_info) */
>> + mulli r14,r14,24 /* 24 == sizeof(elf64_sym) */
>> + add r14,r14,r13 /* elf64_sym[ELF64_R_SYM] */
>> + ld r14,8(r14)
>> + add r0,r0,r14
>> +
>> +7: add r0,r0,r3
>> stdx r0,r7,r6
>> - addi r9,r9,24
>> +
>> +8: addi r9,r9,24
>> bdnz 5b
>>
>> -6: blr
>> +9: blr
>>
>> .balign 8
>> p_dyn: .8byte __dynamic_start - 0b
>> p_rela: .8byte __rela_dyn_start - 0b
>> +p_rela_end: .8byte __rela_dyn_end - 0b
>> +p_sym: .8byte __dynamic_symtab - 0b
>> p_st: .8byte _stext - 0b
>>
>> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
>> index 2bcca818136a..e9d9bda3ffaf 100644
>> --- a/arch/powerpc/kernel/vmlinux.lds.S
>> +++ b/arch/powerpc/kernel/vmlinux.lds.S
>> @@ -281,9 +281,7 @@ SECTIONS
>> . = ALIGN(8);
>> .dynsym : AT(ADDR(.dynsym) - LOAD_OFFSET)
>> {
>> -#ifdef CONFIG_PPC32
>> __dynamic_symtab = .;
>> -#endif
>> *(.dynsym)
>> }
>> .dynstr : AT(ADDR(.dynstr) - LOAD_OFFSET) { *(.dynstr) }
>> @@ -299,6 +297,7 @@ SECTIONS
>> {
>> __rela_dyn_start = .;
>> *(.rela*)
>> + __rela_dyn_end = .;
>> }
>> #endif
>> /* .exit.data is discarded at runtime, not link time,
>> diff --git a/arch/powerpc/tools/relocs_check.sh b/arch/powerpc/tools/relocs_check.sh
>> index 014e00e74d2b..956b9e236a60 100755
>> --- a/arch/powerpc/tools/relocs_check.sh
>> +++ b/arch/powerpc/tools/relocs_check.sh
>> @@ -54,9 +54,3 @@ fi
>> num_bad=$(echo "$bad_relocs" | wc -l)
>> echo "WARNING: $num_bad bad relocations"
>> echo "$bad_relocs"
>> -
>> -# If we see this type of relocation it's an idication that
>> -# we /may/ be using an old version of binutils.
>> -if echo "$bad_relocs" | grep -q -F -w R_PPC64_UADDR64; then
>> - echo "WARNING: You need at least binutils >= 2.19 to build a CONFIG_RELOCATABLE kernel"
>> -fi
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